1. Field of the Invention
The present invention relates to a zoom lens system, more particularly, to a zoom lens system having a refractive power layout of a negative-positive-positive type or a negative-positive-negative type, and an image pickup apparatus using the same.
2. Description of the Related Art
In recent years, miniaturization of digital still cameras and mounting of image pickup functions onto cellular phones have been advanced. In consequence, it is demanded to further miniaturize and thin image pickup optical systems.
As these image pickup optical systems, zoom lens systems having a zoom ratio above 2.5× are demanded.
As methods for realizing the thin zoom lens systems, there are known a method of disposing a reflective member in the zoom lens system to bend the optical axis as much as 90 degrees, and a method of diverting a part of lens units constituting the zoom lens system to the outside of the optical axis during non-use.
However, the method of bending the optical axis by use of the reflective member requires a space for bending a ray and a space for moving the lens unit to secure the zoom ratio. Moreover, these spaces are not eliminated even when an image pickup apparatus such as the camera is not used, and in consequence, the method is disadvantageous to reduction of the volume of the image pickup apparatus when unused. In addition, owing to the bending of the optical axis, layout in the image pickup apparatus is limited.
On the other hand, in the method of diverting a part of the lens units when unused, a mechanism to divert the lens units is added. Therefore, it is difficult to suppress an adverse influence in a case where the lens unit is decentered from the optical axis. Since driving means for diverting a part of the lens units is required, it is difficult to suppress the volume of the apparatus when unused. This method is also disadvantageous in point of costs.
Furthermore, as types of refractive power layout of the lens units constituting the zoom lens system of a usual collapsible type in which the miniaturization is intended, there are known a zoom lens system of a negative-positive type; a zoom lens system of a negative-positive-negative type; and a zoom lens system of a negative-positive-positive type.
Among them, the zoom lens system of the negative-positive type has the reduced number of the lens units, and hence it is advantageous in reducing total thickness of the lens barrel which directly holds lenses. However, in order to secure the zoom ratio, the second lens unit is moved in the range where the image forming magnification of the second lens unit is one fold. In the case where the image forming magnification of the second lens unit is one fold, the image position moves toward the rear side even when the second lens unit moves toward the object side or the image side. Therefore, the second lens unit cannot be used as a focusing lens unit. In consequence, the first lens unit or the whole zoom lens system is moved for focusing, and an increase in total length of the lens barrel including a focusing mechanism is caused. As a result, this type is disadvantageous in thinning the lens system and securing the zoom ratio.
On the other hand, the zoom lens system of the negative-positive-negative type and the zoom lens system of the negative-positive-positive type are advantageous in that the increase of the total length can be suppressed by performing focusing by the third lens unit.
Furthermore, the zoom lens system of the negative-positive-negative type is advantageous to the miniaturization, because it is possible to reduce the diameter of the front lens (foremost lens of the zoom lens system).
In addition, the zoom lens system of the negative-positive-positive type has a merit that a stable optical performance can easily be obtained.
As the zoom lens system of the negative-positive-positive type or the negative-positive-negative type, a zoom lens system is known in which the third lens unit moves to a position closer to the image side in the telephoto end than in the wide-angle end or hardly moves. However, in such a mode of movement of the lens unit, since the third lens unit is positioned close to an image surface in the telephoto end, the height of off-axial rays increases in the third lens unit. Therefore, this system is disadvantageous to reduction of the diameter of each lens. Moreover, since the focusing sensitivity (the movement amount of the image surface position at a time when the focusing lens moves as much as a unit movement amount) easily drops, the third lens unit necessarily requires a strong power. Since the diameter of the lens is large, the thickness also easily increases.
Furthermore, examples of the zoom lens system in which the third lens unit moves toward the object side during zooming toward the telephoto end are disclosed in Japanese Patent Application Laid-Open Nos. 2000-284177 and 2001-242378, Japanese Patent Nos. 3,513,369 (Examples 3 and 4) and No. 3,606,548 (Example 2).
However, in the zoom lens system described in Japanese Patent No. 3,513,369, since the total length of the zoom lens system is large, and the thickness of each lens unit is also large, the constitution is disadvantageous in thinning the system in a collapsed state. Moreover, it is not possible to secure a sufficient space for moving the third lens unit to perform the focusing in the telephoto end.
Furthermore, in the zoom lens systems described in Japanese Patent Application Laid-Open Nos. 2000-284177 and 2001-242378 and Japanese Patent No. 3,606,548, the second and third lens units are integrally moved during magnification change, and the focusing is performed by moving third lens unit only. However, any of the lens systems is disadvantageous to the thinning, because the total length of the zoom lens system is large, and the thickness of each lens unit is also large.
Furthermore, in the zoom lens system of the negative-positive-positive type or the negative-positive-negative type, the angle of field is easily secured in the wide-angle end, an exit pupil is easily disposed away from an image sensor, and the digital camera can be constituted to be compact. Moreover, the focusing method of moving the third lens unit is usually used to miniaturize the zoom lens system.
In the compact digital camera, an image forming state (so-called contrast peak) in which a modulation transfer function (MTF) value becomes large is calculated from the degree of change of the image forming state on the image surface of the image sensor caused by movement of the focusing lens unit during the focusing operation.
On the other hand, when the third lens unit is moved to thereby perform the focusing operation, in the wide-angle end and the telephoto end, the focusing sensitivity changes with the position where the third lens unit is disposed (the higher the focusing sensitivity is, the faster the focusing operation becomes).
Furthermore, since the F-number in the wide-angle end is different from that in the telephoto end, the depth of focus differs (the smaller the depth of focus is, the faster the focusing operation becomes).
In general, in the compact zoom lens system, the closer to the telephoto end the position is, the larger the F-number becomes, the depth of focus increases, and the speed of the focusing operation lowers.